LibDSP: Add IMDCT

This implements a generic IMDCT to be used by the MP3 decoder.

Userland/Libraries/LibDSP/MDCT.h

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2021, Arne Elster <arne@elster.li>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Array.h>
+#include <AK/Math.h>
+#include <AK/Span.h>
+
+namespace LibDSP {
+
+template<size_t N>
+requires(N % 2 == 0) class MDCT {
+public:
+    constexpr MDCT()
+    {
+        for (size_t n = 0; n < N; n++) {
+            for (size_t k = 0; k < N / 2; k++) {
+                m_phi[n][k] = AK::cos(AK::Pi<double> / (2 * N) * (2 * n + 1 + N / 2.0) * (2 * k + 1));
+            }
+        }
+    }
+
+    void transform(Span<double const> data, Span<double> output)
+    {
+        assert(N == 2 * data.size());
+        assert(N == output.size());
+        for (size_t n = 0; n < N; n++) {
+            output[n] = 0;
+            for (size_t k = 0; k < N / 2; k++) {
+                output[n] += data[k] * m_phi[n][k];
+            }
+        }
+    }
+
+private:
+    Array<Array<double, N / 2>, N> m_phi;
+};
+
+}